Thermal treatment of aluminum-magnesium alloy for improvement of stress-corrosion properties

ABSTRACT

A PROCESS FOR INCREASING THE STRESS CORROSION RESISTANCE OF THE ALUMIUM-MAGNESIUM ALLOYS WHICH COMPRISES PROVIDING AN ALUMINUM BASE ALLOY IN REDUCED FORM CONTAINING FROM 3.0% TO 10.0% MAGNESIUM, BALANCE ESSENTIALLY ALUMINUM, HEATING SAID ALLOY AT A RATE OF 50*F. PER HOUR OR LESS TO A TEMPERATURE OF 600* TO 800*F., HOLDING SAID ALLOYS AT SAID TEMPERATURE FOR 5 MINUTES TO 24 HOURS, AND COOLING SAID ALLOYS AT A RATE 500*F. PER HOUR OR LESS.

United States Patent 3,582,406 THERMAL TREATMENT OF ALUMINUM-MAG- NESIUM ALLOY FOR IMPROVEMENT OF STRESS-CORROSION PROPERTIES Francis P. Ford, Hamden, Conn., assignor to Olin Mathieson Chemical Corporation No Drawing. Filed Oct. 30, 1968, Ser. No. 772,027 Int. Cl. C22f 1/04 U.S. Cl. 14812.7 4 Claims ABSTRACT OF THE DISCLOSURE A process for increasing the stress corrosion resistance of the aluminum-magnesium alloys which comprises providing an aluminum base alloy in reduced form containing from 3.0% to 10.0% magnesium, balance essentially aluminum, heating said alloy at a rate of 50 F. per hour or less to a temperature of 600 to 800 F., holding said alloys at said temperature for 5 minutes to 24 hours, and cooling said alloys at a rate of 500 F. per hour or less.

The present invention relates to a new and improved method of producing aluminum base alloys containing magnesium and the alloy produced thereby. More particularly, the present invention resides in aluminum base alloys containing from about 3.0 to about magnesium and characterized by improved stress corrosion resistance.

The advantages to be derived from alloying magnesium with aluminum base alloys were recognized very early in the development of aluminum technology. Consequently, the aluminum-magnesium series of alloys is one of the oldest used commercially.

It is well known however, that magnesium in aluminum base alloys if present in an amount more than about 3% sensitizes the alloy to stress corrosion. Retention of magnesium in solid solution is readily achieved by annealing the alloy at a temperature above the solvus temperature and cooling at a rate rapid enough to prevent precipitation of a magnesium rich second phase. The alloy may then be cold worked to final gage. However, due to natural aging, magnesium retained in solid solution by the rapid cool tends to precipitate preferentially in the grain boundaries as an aluminum-magnesium intermetallic com pound, thus sensitizing the alloy to stress corrosion. Furthermore, the mechanical properties of the cold worked alloy tend to degrade during service due to thermal recovery, which also occurs at or near ambient temperature.

In order to prevent degradation of the mechanical properties, it is necessary to stabilize the alloy after the final cold working step at a temperature somewhat above that which it will be subjected to in service. Thus, the alloy will not undergo subsequent change of mechanical properties at temperature significantly below the stabilizing temperature.

As is well known, some improvement in resistance to stress corrosion may be obtained if the alloy is slowly cooled, i.e., less than 500 F. per hour, after the final anneal prior to cold working to promote heterogeneous nucleation of the equilibrium magnesium rich phase in the grain matrix as well as in the grain boundaries rather than solely or predominately in the grain boundaries as will occur upon aging of the alloy. The stabilizing treatment, however, in those alloys containing more than 3% magnesium causes additional heterogeneous nucleation of the equilibrium magnesium rich beta phase, or a metastable beta modification, in the grain boundaries and, should the alloy be highly cold worked, at points of three dimensional disregistry in deformation bands.

Precipitation of the aforementioned magnesium rich phase preferentially in the boundaries causes suscepti- 3,582,406 Patented June 1, 1971 ice bility to stress corrosion which increases with increasing magnesium content. As a result, the magnesium content of the aluminum-magnesium alloys is generally limited to about 5.5% magnesium, thus precluding favorable strength properties at magnesium contents in excess of 5.5

Accordingly, it is a principal object of the present invention to provide a new and improved process whereby stress corrosion susceptibility in the aluminum-magnesium alloys is substantially reduced, and the alloy produced thereby.

It is a still further object of the present invention to provide a convenient and expeditious process as aforesaid at reasonable cost.

Further objects and advantages of the present invention will appear hereinafter.

It has been found in accordance with the present invention that the foregoing objects and advantages may be readily achieved by controlling the heating up rate of the last recrystallization anneal prior to the final cold rolling and stabilization steps. The present invention is readily applicable to material which has been hot reduced as well as cold reduced i.e. wherein the grain structure of the alloy has been broken up. The process of the present invention comprises; (A) providing an aluminum base alloy containing from about 3.0% to about 10.0% magnesium, balance essentially aluminum, in hot worked or cold worked form from up to about 95.0% reduction; (B) heating said alloy at a rate of 50 F. per hour or less to a temperature of about 600 F. to 800 F. for about 5 minutes to about 24 hours; (C) holding said alloy at said temperature for 5 minutes to 24 hours, and (D) cooling said alloy at a rate of 500 F. per hour or less down to at least 350 F.

The alloy may then be cold worked and thermally stabilized at a temperature of about 225 F. and 350 F. for about 1 hour to 24 hours in order to prevent softening due to recovery when the alloy is placed in service at ambient temperatures.

Naturally other elements may be present in the aluminum-magnesium alloys as alloying additions or impurities. Common alloying additions may include but are not limited to the following: boron in an amount from 0.001 to 0.350%; chromium in an amount from 0.05 to 0.3%; indium in an amount from 0.002 to 0.80%; gallium in an amount from 0.01 to 0.50%; cadmium in an amount from 0.03 to 0.50%; thorium in an amount from 0.005 to 0.350%; misch metal in an amount from 0.005 to 0.30%; tellurium in an amount from 0.005 to 0.30%; lithium in an amount from 0.01 to 0.80%; germanium in an amount from 0.01 to 0.55%; cobalt in an amount from 0.101 to 0.80%; copper in an amount from 0.10 to 0.60%.

In addition to the foregoing alloying additions, naturally the present invention contemplates the use of the normal impurity levels common to commercial grade aluminum. Impurities may include but are not limited to the following: iron up to 0.50%; silicon up to 0.50% copper up to 0.25%; manganese up to 0.35%; zinc up to 0.2%; titanium up to 0.15%; beryllium up to 0.02%; and others in total up to 0.2%.

As shown in the table hereinafter, it has been found surprising that the aluminum magnesium alloys show an unexpected and remarkable increase in resistance to stress corrosion when processed according to the present invention as compared to a process wherein the heat up rate to the full annealing temperature is in excess of 50 F. per hour. In all cases a slow cooling down rate from the annealing temperature was employed, in the order of 50 F. per hour, since it is well known that high rates of cooling have a detrimental effect on resistance to stress corrosion. This detrimental effect is caused by the tendency of an aluminum-magnesium intermetallic compound to precipitate preferentially in the grain boundaries upon aging of the alloy rather than in the grain matrix, thus sensitizing the alloy to stress corrosion.

The large increase in stress corrosion resistance when a slow heat up rate to the annealing temperature is employed is surprising since it would normally be expected that stress corrosion resistance would decrease with an increased grain size as caused by a slow heat up rate rather than as found with the present invention.

Testing of the alloy as processed according to the present invention was performed by stressing a tensile test specimen of the alloy to 80% of its yield strength in a solution of 6% NaCl+0.005 M NaHCO and applying an anodic current to the specimen of 11 ma./ sq. in. via a platinum gauze cathode. A failure time of 13 hours in 4 EXAMPLE II The alloys of Example I after the treatment of Example I was subjected to stress corrosion tests in the following accelerated manner: Samples 0.060" x 2.0" x 0.25" were stressed at 80 percent of their yield strength in a 6 .percent solution of NaCl+0.005 M NaHCO;;. An anodic current of 11 ma./ sq. in. was applied via a platinum gauze cathode. A failure time of 13 hours in the accelerated tests corresponds to a failure time for preformed U-bend specimens in a marine environment of greater than 3 years, a limit which normally signifies a stress corrosion resistant condition. The results of stress corrosion testing is given in the following table.

TABLE II.-ACCELERATED STRESS CORROSION TESTS 1 All alloys in cold reduced form of about 50%.

Norn.1t may thus be readily seen that the above alloys show a remarkable increase in stress corrosion resistance when a comparatively slow heat up rate of 50 F. per hour to the annealing temperature of 650 F. and 800 F. is employed.

this accelerated test corresponds to a failure time of over 3 years for preformed U-bend specimens in marine environments, the limit normally considered as constituting a stress corrosion resistant condition.

The results of accelerated testing, and the composition of the alloys employed, are given in the following illustrative examples.

EXAMPLE I An alloy having the following composition was prepared from a charge of commercial purity aluminum, master alloys of iron-aluminum, chromium-aluminum, boron-aluminum, beryllium-aluminum and the other alloying additions in elemental form. The alloy was cast in the form of 6.0" x 4.0" x 1.5" ingots.

After casting, the above alloys were scalped to 1.40 inches and homogenized at 950 F. for 16 hours, employing a slow heat up rate of about 50 F. per hour from 750 F. After homogenizing the alloys were furnace cooled and hot rolled at 675 F. to 0.3 inch. The alloys were then cold rolled to about 0.150 inch, and then annealed at various times "and temperatures and employing various heating up rates as shown in Table II. The alloys were then all cooled down to ambient temperature from the annealing temperature, and at a cooling rate of about 350 F. per hour to 350 F. Following the anneal, the alloys were then cold reduced about 60.0% followed by stabilization at various times and temperatures as shown in Table II.

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended to be embraced therein.

What is claimed is:

1. A process for increasing the stress corrosion resistance of aluminum magnesium alloys which comprises:

(A) providing an aluminum base alloy in hot or cold reduced form of up to 95.0% and containing 3.0% to 10.0% magnesium and at least one element selected from the group consisting of boron in an amount from 0.001 to 0.350%; chromium in an amount from 0.05 to 0.3%; indium in an amount from 0.002 to 0.80%; gallium in an amount from 0.01 to 0.50%; cadmium in an amount from 0.03 to 0.50%; thorium in an amount from 0.005 to 0.350%; misch metal in an amount from 0.005 to 0.30%; tellurium in an amount from 0.005 to 0.30%; lithium in an amount from 0.01 to 0.80%; germanium in an amount from 0.01 to 0.55%; cobalt in an amount from 0.10 to 0.80%; copper in an amount from 0.10 to 0.60%, balance essentially aluminum,

(B) heating said alloy at a rate of 50 F. per hour or less to a temperature of 600 to 800 F.,

(C) holding said alloy at said temperature for 5 minutes to 24 hours,

(D) cooling said alloy at a rate of 500 F. per hour or less to 350 F. or less, and

(E) cold working said alloy and stabilizing at a temperature from 225 to 375 F. for one hour to 24 hours after said cooling.

2. The process of claim 1 wherein said alloy contains iron up to 0.50%, silicon up to 0.50%, copper up to 0.25%, manganese up to 0.35%, zirconium up to 0.2%, titanium up to 0.15%, beryllium up to 0.02%, and others in total up to 0.2%.

3. The process of claim 1 wherein said reduction is 3,135,633 6/1964 Hornus 148--159 cold reduction wherein the grain structure of the alloy 3,232,796 2/1966 Anderson 14811.5 has been broken up. 3,346,377 10/1967 Jaga'ciak 14811.5X

4. The process of claim 1 wherein said elements are 3,366,476 1/ 1968 Jagaciak 148-11.5X chromium and boron.

5 CHARLES N. LOVELL, Primary Examiner References Cited US. Cl. X.R.

UNITED STATES PATENTS 148-+11.5, 32, 32.5 2,157,150 5/1939 SOmers 148l59 

